Rearend w/posi upgrade - need advice

69Mach!

New Member
Aug 24, 2005
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I am wanting to put a "better" rearend in my factory original 69 Mach 1. Ratio is high - 3.0, I think - so I am wanting something like 3.50 or 3.70 with posi. Car will be used for occasional cruising, not raced, but I want it "snappy".

First, I don't know if it has the 28 or 31 spline axles. Also, should I buy a complete 3rd member and swap out the old one or buy the necessary parts to build it from scratch?

Either way, I would also want advice on which brands or companies to go with. I see a lot of stuff offered on Ebay, but don't want to go the wrong way.

I am also about to have the engine built. I will go with a fairly stock setup - only adding a cam, carb, intake combination to increase stock hp to something around 320-340 hp (that's another post for another day!).
 
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There's a 90% chance you've got 28 spline axles. The cheapest way to a posi(traction loc) is to pull the chunk from a late 70's to mid 80's Bronco. You'll get 3.50's (most likely ratio you'll find in these) but with 31 spline axles. New axles for your housing will be another $300-$400. The total outlay this route is about $600 to $700
 
Ok, that's cool, but what's the cost/benefit ratio by going to a 31 spline? I always assumed that I could make a strong enough street machine with the 28's.

In other words, if I am going to the trouble and expense of changing the rearend, is it a no-brainer to go ahead and upgrade the axles as well or would that just be a luxury considering my mildly-wild intentions for the car?
 
The only point in the 31's was that's what you'll likely find in the late 70-early 80's Bronco rears. Most of these had traction loc with 3.50-3.70 ratios. They're the most likely to find what you're looking for. Some of the other pickups got T/L too, just not as many.
 
If you plan on making serious power (above 350 hp) and running slicks or drag radials then you MIGHT break 28 spline axies. Otherwise I would not worry about it.

I would go with a 3.50 ratio. If you know a good rear end builder then have him build you one using your existing pumpkin. Otherwise buy a complete third member and install the whole thing. Be careful about buying from "whoever" on ebay - you might get a piece of junk. Shipping is expensive so you only want to do it once.
 
Go with a TruTrac limited slip - they are the cats ass. With the power you want to make and the non-race application you have in mind the upgrade on the axles is wasted money.

Dependant on your tranny I'd think about the 3.70 gears.

Good luck...

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Tru Trac is great. I am running 3:55 with a T5Z and it is a great combo for a daily driver. I autocross a few times a year and the Tru Trac works without any noise or grabbing. I actually got mine from Ebay and it is fine. Check the rating of the seller before you buy. Its also hard to realize any real improvement from a cam without changing the ratio. Look at the cam you are thinking of installing to see is a ratio is recomended.